1. Field of the Invention
This invention relates to an information recording medium having a plurality of recording layers which can be accessed from one side of the medium. An example of such an information recording medium is a two-layer single-sided DVD (digital versatile disc). In addition, this invention relates to a method and an apparatus for recording a signal on an information recording medium. Furthermore, this invention relates to a method and an apparatus for recording and reproducing a signal on and from an information recording medium.
2. Description of the Related Art
A two-layer single-sided DVD has a disc substrate whose one side is formed with a laminate of two recording layers. The two recording layers can be optically accessed from one side of the DVD. A two-layer single-sided DVD of a write-once type uses organic dye films as recording layers. A two-layer single-sided DVD of a rewritable type uses phase change films as recording layers.
To record a signal on a DVD, a recording laser beam modulated in accordance with the signal is applied to the DVD. The quality of the recorded signal on the DVD depends on the power of the recording laser beam applied thereto. In general, a recording layer of a DVD has a power calibration area (PCA). Test recording and reproduction are performed on the DVD before a desired information signal is recorded thereon. During a first stage of the test recording and reproduction, test signals are sequentially recorded on the PCA in the DVD while the power of the recording laser beam is changed among different values. The test signals are assigned to the different powers of the recording laser beam, respectively. During a second stage of the test recording and reproduction, the recorded test signals are reproduced, and the reproduced test signals are evaluated. An optimum power of the recording laser beam is decided on the basis of the results of the evaluation of the reproduced test signals. During the recording of a desired information signal on the DVD which follows the test recording and reproduction, the recording laser beam is controlled at the decided optimum power.
In a two-layer single-sided DVD, the near recording layer is semitransparent while the far recording layer is reflective. During the recording of a signal on the near recording layer, a laser beam is focused thereon. During the recording of a signal on the far recording layer, the recording laser beam is applied thereto through the near recording layer. The application of the recording laser beam to the near recording layer to record a signal thereon changes the physical characteristics of the near recording layer from the original. Therefore, during the recording of a signal on the far recording layer, the intensity of the recording laser beam reaching the far recording layer depends on whether or not a portion of the near recording layer through which the recording laser beam passes has undergone signal recording. This fact makes it difficult to accurately decide an optimum power of the recording laser beam for the far recording layer.
In a two-layer single-sided DVD, the two recording layers are different in eccentricity with respect to the disc center. The eccentricity difference is disadvantageous to test recording and reproduction.